A spectrometer (sometimes referred to as a spectrograph) is an instrument wherein a solid, liquid, or gaseous sample is illuminated (often with non-visible light, such as light in the infrared region of the spectrum), and the light from the sample is then captured and analyzed to reveal information about the characteristics of the sample. As an example, a sample may be illuminated with infrared light having known intensity across a range of wavelengths, and the light transmitted and/or reflected by the specimen can then be captured for comparison to the illuminating light. Review of the captured spectra (i.e., light intensity vs. wavelength data) can illustrate the wavelengths at which the illuminating light was absorbed by the sample, which in turn can yield information about the chemical bonds present in the sample, and thus its composition and other characteristics.
Spectrometers are sometimes incorporated into spectrometric microscopes, which capture spectra from some small desired area of a sample, and which may also provide a visible magnified image of this area. A common problem with spectrometric microscopes is the quality of the illumination: it is desirable to illuminate a region of interest on a sample with very bright light containing the wavelength(s) of interest, with the light being uniform across the entire region. Unfortunately, this is generally difficult to attain. As an example, infrared illumination of a sample stage (a mount or chamber bearing the sample) is often provided by a high-intensity incandescent lamp, wherein the lamp filament is specially selected to emit light having the desired wavelengths, or an arc lamp which emits light of the desired wavelengths by generating an electrical (plasma) arc between a pair of electrodes in a bulb. Neither type of lamp tends to provide uniform illumination across its area: filaments tend to have “hot spots” which glow more brightly and shift location across the filament over time, and arcs tend to have brightness which varies in both time and location between the lamp electrodes. The spatial and/or time variability in lamp brightness can in turn lead to problems with spectrometric measurements, since nonuniform illumination of a sample can make it seem as if the sample's composition varies across its area: different sample areas will provide greater or lesser light, but it will be unknown whether this is owing to light interaction with the sample (e.g., absorption by the sample), or simply owing to irregular illumination.
Illuminating light can effectively be made more uniform by interjecting a pinhole or other aperture between the lamp and the sample stage, or by interposing diffusers such as frosted glass. However, these greatly reduce light transmission to the sample stage, and thus are usually nonideal. Various correction methods have also been developed to account for nonuniform illumination, such as alternating the illuminating light from the sample stage between its measuring detector (an array of photosensitive elements measuring the light from the sample) and a reference detector. Here, the readings from the measuring detector can be compared to those from the reference detector, and can be “normalized” for variations in intensity seen across the reference detector. Unfortunately, such correction methods are also nonideal since nonuniformities can also exist between detectors, leading to degraded resolution in the resulting spectrometric readings. It would therefore be useful to have available additional methods and devices which at least partially address the difficulties caused by nonuniform illumination.